1. Field of the Invention
The invention relates generally to the field of integrated circuit (IC) device packaging technology, and more particularly to ball grid array (BGA) package substrate configurations.
2. Background Art
Integrated circuit (IC) dies are typically mounted in or on a package that is attached to a printed circuit board (PCB). One such type of IC die package is a ball grid array (BGA) package. BGA packages provide for smaller footprints than many other package solutions available today. A BGA package has an array of solder balls located on a bottom external surface of a package substrate. The solder balls are reflowed to attach the package to the PCB. The IC die is mounted to a top surface of the package substrate. Wire bonds typically couple signals in the IC die to the substrate. The substrate has internal routing which electrically couples the IC die signals to the solder balls on the bottom substrate surface.
A number of BGA package substrate types exist, including ceramic, plastic, and tape (also known as “flex”). In some BGA package types, a stiffener may be attached to the substrate to supply planarity and rigidity to the package. In such packages, the IC die may be mounted to the stiffener instead of the substrate. Openings in the stiffener may be used to allow the IC die to be wire-bonded to the substrate.
Die-up and die-down BGA package configurations exist. In die-up BGA packages, the IC die is mounted on a top surface of the substrate or stiffener, opposite of the side to which the solder balls are attached. In die-down BGA packages, the IC die is mounted on a bottom surface of the substrate or stiffener, on the same side as which the solder balls are attached.
Conventional BGA packages are subject to high thermal stresses that result from the heat given off during operation of the mounted IC die. The thermal stresses are primarily imposed on the IC die and solder balls due to the mismatch of the thermal expansion coefficient (CTE) between the semiconductor die and the metal stiffener. As a result, conventional flex BGA packages have difficulty in meeting reliability requirements for die sizes larger than 9 mm. See, e.g., Thompson, T., et al., Reliability Assessment of a Thin (Flex) BGA Using a Polyimide Tape Substrate, International Electronics Manufacturing Technology Symposium, IEEE, pp. 207-213 (1999).
The tape substrate used in flex BGA packages is typically polyimide, which has very low values of thermal conductivity. Consequently, the IC die is separated from the PCB internally by the tape substrate thermal barrier. The lack of direct thermal connection from IC die to PCB leads to relatively high resistance to heat transfer from IC die-to-board (theta-jb).
A stiffener attached to a substrate enhances heat spreading. However, the openings on the stiffener/heat spreader for wire bond connections tend to reduce the thermal connections between the IC die and the edges of the stiffener/heat spreader. As a result, heat spreading is limited largely to the region of the IC die attach pad, while areas at the stiffener/heat spreader periphery do not contribute effectively to heat spreading.
Dissimilar materials are used to build flex BGA packages. These materials expand and contract at different rates due to changes in temperature during manufacturing of the package, and application of the package. The CTE of copper typically used for a stiffener/heat spreader in a tape BGA package is approximately 17.4×10−6/° C. For polyimide tape, the CTE may vary from 30×10−6/° C. to 66×10−6/° C. Because of the difference in CTE values, changes in temperature during the BGA package assembly process, testing, and application may lead to high levels of thermal stress. De-lamination of the polyimide tape substrate from the stiffener/heat spreader may occur when the adhesive film between the tape and the stiffener/heat spreader can not accommodate the thermal stress during the solder reflow process, thermal fatigue test, and temperature storage test, for example.
Furthermore, because of the high density of the BGA package substrate routing circuitry, it is difficult to bond each power and ground pad on the IC die to the substrate by a corresponding bond finger. As a result, the distribution of ground and power signals connecting to the IC die is frequently compromised in conventional BGA packages.
Hence, what is needed are BGA packages with improved heat spreading capabilities, while also providing for greater routing capacity and higher levels of IC electrical performance.